1. Technical Field of the Invention
The present invention relates to a positional information recorder which records positional information, a positional information recording method, and an imaging device having the positional information recorder.
2. Description of the Related Art
A positional information recorder, which receives radio waves, measures a present geographical point based on radio waves, calculates positional information, and records the positional information, often uses GNSS (Global Navigation Satellite System), which measures a position with radio waves from a plurality of satellites. The use of a system with GNSS is not limited to vehicle equipment such as car navigation systems. It has been used in portable devices, cell-phones and the like in recent years.
Moreover, imaging devices such as digital cameras have GNSS. In such imaging devices, shot image data in which geographical positional information at the shot point is stored can be recorded. GPS (Global Positioning System) is well-known among positioning systems with GNSS.
Furthermore, systems which measure a present point by using wireless signals from a wireless signal output device disposed in a rod structure without using radio waves from satellites are known.
These systems have a common feature which measures a position by receiving radio waves.
In order to calculate positional information in GNSS, it is necessary to receive radio waves from a plurality of satellites. For this reason, in a poor radio wave receiving point, the position can not be measured. For example, in tunnels and basements, radio waves can not be sufficiently received, so that the positions can not be measured. This is a common problem for not only GNSS but also positioning systems using radio waves.
Some navigation systems which are installed in, for example, vehicles can estimate positional information at the present point by calculating a moving distance and a moving direction from the last measured point with an acceleration sensor and an angular velocity sensor based on positional information measured by then even in a poor radio wave receiving condition.
The navigation systems are not required to record and store estimated positional information because even if the estimated positional information includes some errors, the positional information at the present point is corrected by accurate positional information when the radio wave receiving condition is stabilized.
On the other hand, imaging devices such as digital cameras are required to record positional information at a shot point together with image data. For this reason, it is necessary to record the positional information at that point even in a poor radio wave receiving condition. Moreover, if estimated positional information is recorded, it is desirable to convert the estimated positional information into accurate positional information when the radio wave receiving condition is improved.
More specifically, it is desirable for a positional information recorder which is installed in a device which uses positional information by recording the positional information such as an imaging device to estimate positional information in a poor radio wave receiving condition, and to convert the estimated positional information into accurate positional information when the radio wave receiving condition is improved.
As a standard which records image data associated with positional information, “Image File Format Standard for Digital Still Camera Exif 2.21” (hereinafter, referred to as EXIF Standard) revised in September, 2009 by the Electronics Standards Information Industry Association is known (refer to, for example, JEITA CP-3451A Image File Format Standard for Digital Still Camera Exif 2.21 consolidated edition revised in September, 2009 issued by the Electronics Standards Information Industry Association).
The positional information stored by the EXIF standard is referred to as GEOTAG. GEOTAG is used to express positional information calculated by GNSS installed in an imaging device as the combination of latitude, longitude and altitude, and this is stored in a header portion of an image file.
None of the prior art documents describe the purpose of solving the above-described technical problem, namely, a method of converting positional information recorded in a poor radio wave receiving condition into accurate positional information in a positional information recorder which is installed in a portable use device such as an imaging device. However, Japan Patent Publication No. 4312566 was found as a prior art document associated with the present invention. The invention described in Japan Patent Publication No. 4312566 provides an imaging device having a direction detector which detects a geographical direction of a standard direction of a light receiving surface of an imaging element according to a shooting mode, and the imaging device can record imaging data associated with direction information.
Even if relative positional information can be recorded in a portable device such as an imaging device as a substitute in a poor radio wave receiving condition, if it can not be converted into accurate positional information (positional information by positioning) after that, inaccurate positional information is recorded. Therefore, such a device is inconvenient for a user.
GNSS causes several seconds of time lag until it realizes a positional information calculable condition from activation, namely, a positionable condition by receiving a plurality of radio waves.
This is because a time is required for a process which confirms normal receiving of the predetermined number of radio waves and a process which calculates the present point by the received radio waves, in order to accurately calculate a geographical position (latitude, longitude and altitude).
More specifically, even if an imaging device is in a shootable condition by turning on a power source, if an initialization process of the installed GNSS is not completed, the positional information can not be recorded. If image data is recorded together with the positional information, the shooting can not be started until GNSS realizes a positionable condition after it is completely activated.